1. Field of the Invention
The present invention relates to a radiation imaging apparatus suitable to be used for a medical diagnosis, an industrial non-destructive inspection, and more particularly to a radiation imaging apparatus and a radiation imaging system suitable for radiography for obtaining a dynamic image. Hereupon, the radiation is not limited to X-rays, but includes α-rays, β-rays, γ-rays and the like.
2. Description of Related Art
Conventional X-ray imaging systems include a film radiographing system and a digital radiographing system. The film radiographing system irradiates a patient with X-rays to expose a film with the X-rays which have penetrated the patient. The digital radiographing system converts the X-rays which have penetrated the patient into electric signals, and detects the electric signals as digital values with an AD converter to take the digital values into a memory. The present main current of the latter system is a system for performing digitalization by accumulating X-ray images in photostimulable phosphor called as an imaging plate (hereinafter referred to as an “IP”), the representative material of which is BaFBr:Eu, and after that by scanning the IP with laser beams to obtain visible rays from the IP. The system, then, converts the obtained visible rays from the IP into electric signals with a photomultiplier tube or the like to digitize the electric signals.
Another digitalization system has lately been put to practical use. In the digitalization system, X-rays irradiate X-ray visible conversion phosphor using Gd2O2S:Tb or CsI:Tl as the representative material thereof, and visible rays are emitted in proportion to the dosage of the X-rays. The emitted visible rays are converted into electric signals by an amorphous silicon photosensor to be digitized. An element for implementing the digitization system is called as a flat panel detector (hereinafter referred to as a “FPD”). A type of the FPD does not use the X-ray visible conversion phosphor, but uses Se or PbI2 as a material to absorb X-rays directly and to convert the absorbed X-rays into electric signals.
In addition, there is a further apparatus. The apparatus irradiates a primary phosphor with X-rays, and accelerates and focuses photoelectrons emitted from the fluorescent screen of the primary phosphor with an electron lens onto a secondary fluorescent screen. Then, the apparatus converts a fluorescent image (an X-ray image) on the secondary fluorescent screen into electric signals with an image pickup tube or charge coupled elements (CCD's). The apparatus is called an image intensifier (I.I.), and is generally used for fluorography. Also by this apparatus, electric signals can be detected as digital values, and the system adopted by this apparatus is one of the digital radiographing systems.
As described above, a variety of apparatus for digitalizing X-ray images exist. The demands for digitalization at medical scenes have recently been increasing more and more. The digitalization of image data could bring about the advantage of the easiness of the recording, the displaying, the printing and the holding of radiographed data. Moreover, the image processing of radiographed data using a computer enables a doctor to be aided at the time of a diagnosis by the interpretation of a radiogram. Moreover, it is said that an automatic diagnosis only by a computer without any doctors for the interpretation of a radiogram will be able to be realized in the near future.
Even at current medical scenes in which the radiation imaging apparatus are shifting from the film radiographing system, the so-called analogy radiographing system, to the aforesaid digital radiographing system, simple X-ray radiography is performed as a first step of X-ray photography. For example, in case of a chest region, the X-ray radiography is called as chest simple X-ray radiography. The chest simple X-ray radiography performs the X-ray radiography of the front face (or the side face) of the chest region of a human body. It is said that a radiographing area of the so-called half size (35 cm×43 cm) or more, possibly the size of 43 cm×43 cm or more, is generally necessary for covering the whole area of the chest region (the upper half) of a human body. For the chest simple X-ray radiography, the digital radiographing system using the FPD is more expected in the future than the one using the I.I., which is considered to have a problem of distortion at peripheral parts of an image.
The chest simple X-ray radiography can expose the information of a body in the vicinity of the lung field of the upper half of the body, such as the information of an esophagus, a trachea, pulmonary blood vessels, alveoli, a heart, heart blood vessels, a diaphragm, costae and clavicles, by X-ray radiography for a radiograph at one time. Accordingly, the chest simple X-ray radiography is frequently performed as the radiography effective for the screening of foci. However, penetrated images are observed on the principle thereof in the radiography. Consequently, when a focus exists behind such a tissue as a costa, heart blood vessels, or a diaphragm, the focus doubly overlaps the tissue in a penetrated image. Hence, there are some cases where it is difficult to find (or discover) the shadow of the focus. Thereby, the chest simple X-ray radiography has the problems that the efficiency of the screening of foci decreases, and that the discovery of the foci is retarded.
As means for solving the problems, a method has been realized. In the method, two imaging plates (IP's) are used for radiography at two times under different tube voltages, and the shadow of a bone part is removed by performing a subtraction process between X-ray images obtained from the two IP's. The method is called as an energy subtraction process (ES process). The method is the radiography using the phenomenon such that, when the energy of X-rays is changed, the absorption degrees of the energy by bone tissues are different from those by soft tissues such as blood vessels, lymphatic vessels and nerves.
As an example of the ES process, Japanese Patent Application Laid-Open No. H2-273873 discloses the radiography, in which the subtraction of images radiographed with radiation rays irradiated from a plurality of radiation sources having different radiation energy from one another is performed after the distortion of the images has been corrected on the image signals of the images. Moreover, Japanese Patent Application Laid-Open No. H3-106343 discloses the configuration in which a dual energy generation mechanism is formed on the X-ray irradiation port of an X-ray tube to generate X-rays having different energy from one another at the timing of image collection. Moreover, Japanese Patent Application Laid-Open No. H3-133276 discloses a method of displaying energy subtraction images by obtaining only the images of affected tissues as difference signals to perform display with the images of the affected tissues being added as the depth information at the third dimension. Moreover, Japanese Patent Application Laid-Open No. H5-260382 discloses a configuration for performing a subtraction by recording images radiographed with X-rays having different energy at different parts on a fluorescent sheet.
However, although the ES process is effective in the point of view of the removal of bone shadows, but the bone shadows are not always removed completely. The ES process particularly has the problem that the bone shadows remain to some figures and physiques of patients and to some kinds of foci. Moreover, foci do not always exist, for example, behind costae. When a focus exists behind a diaphragm or a heart, there is the remaining problem that the ES process for the removal of bone shadows is insufficient to some patient's conditions (physiques, foci) when the ES process is used as the only removal method. Moreover, there is room for further examination of suitable combination methods of the subtraction process and dynamic image display.